Visualization of Flame Propagation by Laser-Fluorescence Imaging of Nitrogen Dioxide

Abstract

Laser induced fluorescence (LIF) from a trace concentration of nitrogen dioxide (NO₂) molecules (700 ppm) was used to visualize the propagation of a flame in a stoichiometric methane-air mixture in a constant volume combustion chamber. The fluorescence excitation was produced by a sheet (45 × 1 mm) of laser light from the second-harmonic output (532 nm) of a pulsed Nd : YAG laser. Two-dimensional detection of the fluorescence from the NO₂ molecules was provided by a gated, image-intensified vidicon camera read in a 100 × 100 pixel format with an effective spatial resolution of 0·6 × 0·6 mm. With 0·1 joules of energy in a single laser pulse the NO₂ detectability was 70 ppm at minimum signal-to-noise (S/N= 1). In the absence of combustion the background noise level was limited to the variation in the fixed pattern noise of the detector. Although the vidicon was electronically gated and provided a signal rejection of 10⁻⁶, during combustion the chemiluminescence from the flame completely masked the LIF signal from NO₂. Additional gating of the vidicon camera with a mechanical shutter was required to suppress the chemiluminescence contribution from the flame. With the effective removal of the flame emission from the LIF signal during combustion, time-resolved visualization of flame propagation was obtained from the disappearance of the fluorescence signal as the NO₂ molecules were consumed in the flame.